Why would it not be possible to put GE90-115b engines on say a aerodynamically cleaned up 757, and get it to fly supersonic? I don't understand why if GE90s can get a 777 to M.87, why wouldn't they be able to get a smaller aircraft supersonic?

I think the issue is probably with the design of the engine. I could be wrong, but I think funny things start happening when you have a high-bypass turbofan like the GE90, or any other modern jet engine seen on new airliners(CFM56, PW4082, etc...) going supersonic.

Around Mach 1 drag increases drastically, and if your aircraft isn't designed for supersonic flight it you'll have trouble going any faster. Shocks will start to appear and in the engine inlets they will cause pressure losses that affect engine performance. Also, one of the variables that set an aircrafts maximum cruising speed is the critical Mach number, at this speed flow starts going supersonic over the top surface of the wing creating more drag. Any faster and all sorts of funny things start to happen.

We all wish it were that easy, but it's not just a question of power. The earlier Learjets had enough power to go supersonic, but the tip tanks would fall off if you tried it. (followed by the wings, of course) If I remember correctly, the venturi effect starts to work backwards above Mach 1, therefore you have to have complex engine inlets to slow the airflow back down so the engine keeps working normally.

The issues of fuselage/wing aerodynamics and propulsion (in relation to exceeding the speed of sound) are two different subjects. As far as I understand the original poster, the hypothetical situation that was set-up focused on the propulsion aspect and assumed that the other concerns were taken care of.

So, in that light - the main design 'flaw' limiting the GE90 as a subsonic engine is the inlet design, as mentioned already. The air entering the engine must be slowed down to avoid shocks associated with supersonic flow to form. If these shocks do form, drag increases greatly and engine efficiency plummets. In addition, supersonic flow entering the engine raises the temperature and stress the engine must withstand. The GE90's inlet is not intended to slow down the entering airflow as much as is necessary to mitigate the aforementioned problems (among others), and thus it is not a feasible engine for supersonic flight.

The inlet for the fan would have to slow the supersonic air down to subsonic. This is accomplished with a convergent-divergent duct. With supersonic flow the C-D duct will slow the air down to subsonic. This is done on supersonic jet aircraft which use turbofan engines such as the F/A-18.

Laxx5. I lost you. But then not being technical I am lost easily. Concorde had the gates or flap that came down infront of the engine- that concept I understand. How is it that fighter jets like the F18 you refer to could go supersonic and not have "gates"? They have ducts that open up and split the air and thus slowing it down?

there is economy class, business class, first class...then Concorde..pure class

All supersonic aircraft, fighter jets, Concorde, etc, must have variable geometry inlet ducts which can change, adjust the speed of incoming air, too below supersonic when at supersonic speed and to speed up air to optimum speed when the aircraft is subsonic. These inlet ducts control the air to what the engine requires. A good example is the nose cones on the front of the SR-71. Since that aircraft is capable of Mach 3+, the inlet duct nose cones are an extreme example. To put it simply, a turbofan engine can only operate efficiently with incoming air which is subsonic.

Thanks guys...that makes it a lot clearer. As Quantas A332 points out, I was focusing on the propulsion aspect of supersonic flight rather than the aerodynamics / structure of the actual aircraft. Would I be right in assuming then that if air is slowed down to subsonic speeds before / as it entered the engine, this could be possible?

All supersonic aircraft, fighter jets, Concorde, etc, must have variable geometry inlet ducts which can change, adjust the speed of incoming air, too below supersonic when at supersonic speed and to speed up air to optimum speed when the aircraft is subsonic.

Not all supersonic aircraft have variable geometry inlets. The inlets themselves are, on most modern fighter aircraft, have a fixed inlet guide path to slow down the air to subsonic speed (and to reduce radar signature). Variable geometry can be seen on aircraft such as the Concorde and the B-1 to improve takeoff and subsonic performance, but it adds weight and complexity to the aircraft that most fighters cannot carry with significant drawbacks.

However, on some aircraft, such as the Typhoon and the MiG-29, you may see augmentative devices to help increase flow to the engines at subsonic speeds.

All supersonic aircraft, fighter jets, Concorde, etc, must have variable geometry inlet ducts which can change, adjust the speed of incoming air, too below supersonic when at supersonic speed and to speed up air to optimum speed when the aircraft is subsonic.

Not totally true, as Buckfifty said. Ever get up close to an F-14 or F/A-18? There are vents on top of the intakes, but otherwise, those intakes don't change shape, but have been carefully designed for max flow. I am especially fond of the new Super Hornet intakes, very nicely shaped, but the engines are inboard enough that the compressor blades will not reflect radar energy.

Supersonic inlets are shaped in such a way that there are a series oblique shock waves and then a normal shock. This is essential for supersonic flight because at higher Mach numbers a normal shock will result in enormous pressure losses while a series of oblique shocks will get the Mach number down to just over 1 before a last normal shock terminates the supersonic flow, with losses much smaller. This is why inlets on fighters usually are angled or have an inlet cone to get the shocks where they are wanted.

All the inlet/engine combinations you're talking about here are dependent on the design of the wing/fuselage. As pointed out up above that's not the premise here.

What you need is something evolutionary. If you're going to just slap a new engine and pod on a 757 wing and hope for it to go supersonic everything that engine needs from an aerodynamic standpoint is going to have to be on that pod. You can't count on the airframe (at least if you want to have it resemble a 757 in any shape or form).

So this pod is going to have to do several things: 1. House the massive GE90 engine. 2. Allow for the mass flow of the engine. 3. Compress the airflow down to subsonic speed. Just off the top of my head I'd say this pod is going to be about the size of 737 fuselage.

OK so it's probably not going to fit under our unredesigned 757 wing......

Would probably be smarter to start with an engine that's designed for non-augmented supersonic cruise, say the F22 engine. Hard to say how many we're going to need since our 757 wing is so supersonic unfriendly, but lets go with 6 for starters. Since this engine is considerably smaller than the GE90, we can pod up the engines in a side by side fashion ala B-52 only with 3 to a pod vs 2.

To get the mass flow slowed down before it enters the engine we're going to need a pod say 2.5 times the engine length. I envision the actual engine section being underneath the wing with the inlet section protruding out front.

Ok, now what's this massive flat topped pod going to do when we go to rotate? It's going to blank out a good amount of wing acreage that's what.

Folks you can't take a subsonic optimized airframe and make it supersonic cruise simply by putting a supersonic engine on it. The airframe needs to be optimized for the supersonic cruise condition. Has anyone seen a supersonic airframe where the engine/inlet were not integral to the airframe design? I'm not aware of one, although it's commonplace in subsonic airliner designs.

I wasnt literally asking if a 757 could go supersonic with GE90s. I was trying to understand why simply increasing the thrust to weight ratio on an aircraft (even one that is aerodynamically suitable to supersonic flight) would not be sufficient to achieve stable supersonic flight.

I guess I should explain my flight of fancy a bit more: Concorde (as beautiful as she was) was a loud gas guzzler. The 777 (no A v B here) is often touted as being one of the most efficient aircraft in service. The GE90s, RR Trents etc can accelerate the 777 to M0.87 and maintain that speed. I started wondering, if you were to half the weight and size of the 777 and reshape it for supersonic flight (ie double the power to weight ratio) you would still be unable to use the GE90s in their current form - and I was just wondering why. Thanks to everyone who has replied to this post, as I now have a somewhat better understanding of why this wouldnt be possibe.

TheBigOne: the 777 is aerodynamically extremely efficient... subsonically. Once you start reaching for sonic speed, and beyond, it's a whole different ball game. For lack of a better word, the air behaves differently So while you could probably drive the 777 supersonically by sheer brute force (assuming it didn't come apart in the process) this would not happen efficiently.

"There are no stupid questions, but there are a lot of inquisitive idiots."

DeltaGuy, actually the F-14 has a very complex set of ramps and guide vanes in the inlet to slow the air down and keep the shockwave where they want it. A lot more than just vents on top of the inlet. They are computer programmed for different configurations depending on the speed.

The F-18 doesn't have all that as far as I know, but then again the 18 caps out at M 1.8 while the Tomcat gets up to between 2.2 and 2.4, depending on who you believe (2.2 imo).

I started wondering, if you were to half the weight and size of the 777 and reshape it for supersonic flight (ie double the power to weight ratio) you would still be unable to use the GE90s in their current form - and I was just wondering why. Thanks to everyone who has replied to this post, as I now have a somewhat better understanding of why this wouldnt be possibe.

What you are describing is almost the exact concept of the Boeing Sonic Cruiser.... it was a 767-sized aircraft that would have used 777-sized engines to cruise right at mach .98, subsonic but still fast enough to reduce flight time on long distance pairs.

What you are describing is almost the exact concept of the Boeing Sonic Cruiser.... it was a 767-sized aircraft that would have used 777-sized engines to cruise right at mach .98, subsonic but still fast enough to reduce flight time on long distance pairs.

Yeah, but there would have been significant design differences, not the least of which was a quasi-delta wing.

When considering the requirement for variable intake devices, remember that the fighter aircraft so far mentioned, had the ability to supercruise for minutes, not for hours in the case of Concorde.
The intake system made the whole concept of a SST possible.

To add to what's already being said, consider the B-1A and B-1B aircraft. The mission profile for the B-1A required a supersonic dash to the target. The B-1B version which ended up being produced does not have this requirement, and to simplify the design the variable inlet system was deleted. I don't have the stats for either aircraft with me, but I'm sure you'll find them easily, and you can see the B-1A was capable of Mach 2+, while the B model can't go very far above Mach 1.

Regarding your original question: I'm not sure what you mean by 'aerodinamically cleaned up' 757. When aircraft are designed they are optimized for a particular design condition (or "mission" if you will). Taking Concorde as an example, the design condition was extended cruise at Mach 2, but it also had to satisfy the off-design condition of subsonic and low speed flight. As you probably well know, everything in design is a compromise!

In the case of 757, it was designed for optimum performance at a given speed range (usually around .8-.82 Mach in modern designs of today, although some other types go faster). Above this you encounter what they call the 'Divergence' Mach number, at which drag takes a significant jump and keeps on increasing the closer you get to Mach 1. So trying to push an airframe designed to fly subsonically up to supersonic speeds is not a very efficient thing to do! If you add to that the fact that you need to supply the appropriate powerplant to make it happen, and that you have no control/structural problems to deal with, you can see that in the end you have to scrap that 757 and start with a fresh design!

25 Wingscrubber
: Aswell as a variable geometry air intake you'd also need to bolt an afterburner onto the back of your GE90 in order to accelerate your gas flow beyond

26 WIDEBODYPHOTOG
: A supersonic GE90 is not possible or practical and here is why. The enormous airflow requirements at Mach 2.0 for a GE90 would require an inlet diffus